The aliens are the main villains of the Tokyo Mew Mew universe. In the English dub, they're called Cyniclons (presumably a play on the word cynical), but in the original (and most other dubs) the species went unnamed. They are distant relatives of humans (sometimes theorized to actually be a primitive form of human), but have several key physical differences. These include: long pointed ears, small fangs, slit pupils, and incredibly pale skin (although that is most likely due to the fact that they live underground).
In Tokyo Mew Mew, the aliens are overwhelmingly antagonistic. The only exception is the Blue Knight who serves as Ichigo's protector.
Aliens also have the ability to create and control Chimera Anima (also called Kirema Anima, Predasites, and Chimeri). Chimera Anima are created when jellyfish-like creatures (Parasite Aliens, also called Infusers) controlled by one of the aliens merge with a normal plant, animal, or human and turn them into hostile monsters. Mews seem to be immune to them.
The aliens originally hailed from Earth, but roughly three million years ago they were forced to leave. After a series of disasters (presumably natural disasters, it's never specified) that brought them to the brink of extinction, the aliens were forced to flee the Earth and relocate to an inhospitable unnamed planet. The surface of the new planet was not able to support them, so they were forced to live underground for three million years. They never truly forgot about Earth and eventually returned with the intention of reclaiming it as their home.
Bracketed sections ("[ ]") contain plot and character details that may spoil the story for some. Highlight at your own risk.
In TMMR, the aliens reappear as the story progresses.
More information as updates continue.
Cannolo is a supporting character and Mew ally, belonging to the same race as the canon aliens. [He was initially intended to represent Deep Blue along with Quiche, Pie, Tart and Gateau; however, his powers failed to mature before Deep Blue's awakening and he was left behind on the planet's hostile environment during the events of Tokyo Mew Mew.]
Since the defeat of Deep Blue at the hands of Tokyo Mew Mew the aliens have managed to attain a sense of peace. Using the Mew Aqua brought back by Quiche, Tart and Pie, the aliens were able to not just revive their planet but make it flourish. While most aliens are content with their lives and are thankful towards the Mew Mews, not everyone feels the same.
Deep Blue's followers didn't just disappear after his defeat, some moved on, supporting all the changes, others, however, grew bitter and angry towards humans, still blaming them for all the suffering their people went through.